This was how I used to do it:
Download Ubuntu Desktop ISO to PC.
Burn ISO to CD.
Boot from CD and select Install Ubuntu.
Select to install to 8 GB USB stick AND set GRUB to USB Stick.
The result of this is that if I boot my laptop WITHOUT the USB Stick plugged in, it will boot directly to Windows 7. However, as and when I need to boot to Ubuntu, I just plug in the USB stick and restart the laptop. It will then boot to GRUB which gives me the option of booting to Ubuntu or booting to Windows.
The KEY to all this is to create Ubuntu boot CD but now I'm not able to with 12.10 because the ISO size is more than 700MB. I've read some threads that mention try burning to DVD-R instead. Tried that but didn't work. (Yes, I choose to burn img (ISO) to disc instead of burn file to disc). I'm using Nero Burning ROM app to do the burning.
So how do I create a Ubuntu 12.10 Desktop LiveCD or LiveDVD using the 753MB ISO ?